1. Field of the Invention
The invention relates to the mixing of gases and liquids. More particularly, it relates to the formation and recirculation of gas bubble-liquid mixtures.
2. Description of the Prior Art
A wide variety of techniques and systems are known in the art for achieving the mixing and/or reacting of gases. Thus, surface aerators, jets and impellers within stirred tanks have been employed for such mixing. When the residence time required for a desired mixing or reaction operation is longer than a few minutes, either on a batch or continuous basis, stirred tank reactors (STR) are commonly employed. As disclosed in "A Mixed Gas-Liquid Stirred Tank Reactor," L. M. Litz, CEP, November, 1985, pp. 36-39, gas is normally fed to a sparger at the bottom of a conventional STR, and a flat-bladed Rushton turbine or other such mixer system is used to shear the gas into small bubbles for dispersion in the liquid phase. Axial flow impellers are commonly employed to facilitate gas dissolution. Further dissolution is said to occur as the gas bubbles rise up through the liquid, but undissolved gas that reaches the gas-liquid interface in the upper part of the STR is normally lost.
The Litz publication refers to a new design STR, called an Advanced Gas Reactor (AGR), that enables gas dissolution and chemical reaction rates to be increased, while gas and power consumption is being reduced. In this AGR system, a down-pumping impeller is employed within a draft tube, with a baffle arrangement at the inlet thereof causing the liquid flowing into the top of the draft tube to form vortices. These vortices cause feed gas from the gas phase above the liquid to pass down through the draft tube. Additional information pertaining to the AGR system is contained in the publication, and in the Litz patent, U.S. Pat. No. 4,454,077, dated June 12, 1984.
It is well known to use such mixing systems for the reaction of oxygen containing gases, such as air, with organic liquids to form various oxygenated products. Typical of such processes are the oxidation of cumene to form cumene hydroperoxide useful as an intermediate in the production of phenol, the oxidation of propionaldehyde to propionic acid, the oxidation of cyclohexane to adipic acid, and the like. Similarly, such mixing systems can be used for the reaction of hydrogen with various organic chemicals or other materials, as in the hydrogenation of edible and non edible fats and oils, or of antioxidants, medicinals, aluminum alkyls, and the like.
When such gas-liquid mixing operations are carried out in relatively simple tanks or towers, the feed gas is typically bubbled in near the bottom of the tank. Mechanical agitation means may be employed to improve gas-liquid mass transfer, to improve heat exchange, or to maintain solid catalysts in suspension in the gas liquid reaction mixture. In some cases, a simple bubble column is employed, with the injected gas rising up through the body of liquid in the tank. In any event, the oxygen, hydrogen, chlorine or other reactive gas reacts directly when in bubble form, or dissolves in the liquid and then reacts, or experiences both forms of activity. Unreacted gas may be recirculated within the liquid phase, to some extent, by the flow of the liquid as influenced by impellers, as in a conventional STR configuration, or additionally by the liquid downflow in the AGR system in which a draft tube is employed.
Gas-liquid mixing operations must include provisions for changes in the volume of liquid resulting from the dispersion of gases therein. Accordingly, a free liquid surface with an overhead gas phase is typically provided for in mixing operations of the type described herein.
A serious limitation as to the amount of oxygen that may be fed into any of the reactor systems described above resides in the fact that many organic compounds can burn, or even explode, when their vapor is admixed with an oxygen-rich gas. It should be noted that little, if any, such safety problem exists when small gas bubbles of high oxygen content are dispersed within the liquid phase of such a flammable material. This is because the liquid phase provides a large heat sink capable of absorbing any rapid energy release that would occur if the gas bubbles were to ignite in the liquid at typical gas bubble concentrations of 5-20 volume %. There is, in addition, a concentration of oxygen in the vapor-gas phase below which combustion will not be sustained, i.e. a lower flammability limit. With respect to many organic materials, this lower flammability limit is in the range of about 8 to 12 volume percent. For safety reasons, reactors typically are fed only as much oxygen as will assure that the unreacted gas does not cause the oxygen in the gas phase to exceed the oxygen concentration of said lower flammability limit. A negative consequence of this limitation is that the oxygen content of the gas bubbles typically falls off considerably from a point of injection of the gas near the bottom of the reactor and the discharge of bubbles at the gas liquid interface near the top of the tank. As a result, the reaction rate and the gas dissolution rate, which typically are dependent upon the oxygen concentration in the gas bubbles, can decline substantially from the bottom to the top of a conventional reactor. It is well known, however, that, in some cases, it would be desirable to carry out a reaction at fairly high gas bubble oxygen concentrations, even up to pure oxygen feed gas, so as to increase the reaction rate or to improve reaction selectivity to the desired reaction rather than to undesired side reactions.
In the AGR system, as pointed out in the Litz publication, the gas that escapes into the gas phase is recirculated back into the liquid via the vortex ingestion path. While the AGR system can thus be employed to overcome the problems associated with loss of gas e.g. oxygen, into the vapor-gas phase, those skilled in the art will appreciate that the gas introduction rate of the AGR system is a function of the vortex characteristics of the system. While the AGR system provides highly desirable process and apparatus for mixing a gas and a liquid, it will thus be seen to require particular design expertise, especially with respect to the application thereof to operations in which a potentially explosive gas mixture could be created in the vapor-gas phase upon the carrying out of particular gas-liquid reactions at desirably high reaction rates. This is particularly so in light of the sensitivity of the AGR system to liquid level changes and of the fact that the vapor phase concentration of oxygen is in equilibrium with the concentration thereof in the liquid phase.
For the reasons above, there is a desire in the art for further gas liquid mixing development, particularly with respect to reactions subject to the potential of gas phase burning or explosions at desirable reaction rates using the reactor systems and techniques known in the art. Such further development would not replace the AGR approach, but would provide an alternative thereto, one not dependent upon the vortex characteristics, liquid level sensitivities and other pertinent factors relating to the AGR approach. While the concerns with respect to potentially explosive vapor-gas phase mixtures in oxygen applications do not generally pertain in hydrogen, chlorine and other gas applications, it is nevertheless desirable in the art to have an alternative approach available also for use in particular non-oxygen gas-liquid applications. Thus, in some instances, it may be desirable to carry out particular reactions in a manner not sensitive to variations in the liquid level existing within the reactor tank. It is also desirable in some instances to assure retention of the feed gas within the reactor volume by virtue of the system design, apart from the characteristics of a vortex as in AGR processing.
It is an object of the invention, therefore, to provide a process and apparatus for the mixing of a gas and liquid without appreciable loss of gas into the overhead gas phase.
It is another object of the invention to provide a process and apparatus for the enhanced mixing of gas and liquid by the use of a draft tube configuration without undue sensitivity to changes in the level of liquid during the processing operation.
It is a further object of the invention to provide a process and apparatus for the reaction of oxygen with organic liquids in which the concentration of oxygen in the vapor-gas phase is maintained below the lower flammability limit pertaining to said organic liquids.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.